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"""Provide access to settings for globally used Azure configuration values.
"""

from collections import namedtuple
from enum import Enum
import logging
import os
import sys
import six
from azure.core.tracing import AbstractSpan

try:
    from typing import Type, Optional, Dict, Callable, cast, TYPE_CHECKING
except ImportError:
    TYPE_CHECKING = False

if TYPE_CHECKING:
    from typing import Any, Union
    try:
        # pylint:disable=unused-import
        from azure.core.tracing.ext.opencensus_span import OpenCensusSpan  # pylint:disable=redefined-outer-name
    except ImportError:
        pass

__all__ = ("settings", "Settings")


# https://www.python.org/dev/peps/pep-0484/#support-for-singleton-types-in-unions
class _Unset(Enum):
    token = 0
_unset = _Unset.token


def convert_bool(value):
    # type: (Union[str, bool]) -> bool
    """Convert a string to True or False

    If a boolean is passed in, it is returned as-is. Otherwise the function
    maps the following strings, ignoring case:

    * "yes", "1", "on" -> True
    " "no", "0", "off" -> False

    :param value: the value to convert
    :type value: string
    :returns: int
    :raises ValueError: If conversion to bool fails

    """
    if value in (True, False):
        return cast(bool, value)
    val = cast(str, value).lower()
    if val in ["yes", "1", "on", "true", "True"]:
        return True
    if val in ["no", "0", "off", "false", "False"]:
        return False
    raise ValueError("Cannot convert {} to boolean value".format(value))


_levels = {
    "CRITICAL": logging.CRITICAL,
    "ERROR": logging.ERROR,
    "WARNING": logging.WARNING,
    "INFO": logging.INFO,
    "DEBUG": logging.DEBUG,
}


def convert_logging(value):
    # type: (Union[str, int]) -> int
    """Convert a string to a Python logging level

    If a log level is passed in, it is returned as-is. Otherwise the function
    understands the following strings, ignoring case:

    * "critical"
    * "error"
    * "warning"
    * "info"
    * "debug"

    :param value: the value to convert
    :type value: string
    :returns: int
    :raises ValueError: If conversion to log level fails

    """
    if value in set(_levels.values()):
        return cast(int, value)
    val = cast(str, value).upper()
    level = _levels.get(val)
    if not level:
        raise ValueError("Cannot convert {} to log level, valid values are: {}".format(value, ", ".join(_levels)))
    return level


def get_opencensus_span():
    # type: () -> Optional[Type[AbstractSpan]]
    """Returns the OpenCensusSpan if opencensus is installed else returns None"""
    try:
        from azure.core.tracing.ext.opencensus_span import OpenCensusSpan  # pylint:disable=redefined-outer-name

        return OpenCensusSpan
    except ImportError:
        return None


def get_opencensus_span_if_opencensus_is_imported():
    # type: () -> Optional[Type[AbstractSpan]]
    if "opencensus" not in sys.modules:
        return None
    return get_opencensus_span()


_tracing_implementation_dict = {
    "opencensus": get_opencensus_span
}  # type: Dict[str, Callable[[], Optional[Type[AbstractSpan]]]]


def convert_tracing_impl(value):
    # type: (Union[str, Type[AbstractSpan]]) -> Optional[Type[AbstractSpan]]
    """Convert a string to AbstractSpan

    If a AbstractSpan is passed in, it is returned as-is. Otherwise the function
    understands the following strings, ignoring case:

    * "opencensus"

    :param value: the value to convert
    :type value: string
    :returns: AbstractSpan
    :raises ValueError: If conversion to AbstractSpan fails

    """
    if value is None:
        return get_opencensus_span_if_opencensus_is_imported()

    if not isinstance(value, six.string_types):
        value = cast(Type[AbstractSpan], value)
        return value

    value = cast(str, value)  # mypy clarity
    value = value.lower()
    get_wrapper_class = _tracing_implementation_dict.get(value, lambda: _unset)
    wrapper_class = get_wrapper_class()  # type: Union[None, _Unset, Type[AbstractSpan]]
    if wrapper_class is _unset:
        raise ValueError(
            "Cannot convert {} to AbstractSpan, valid values are: {}".format(
                value, ", ".join(_tracing_implementation_dict)
            )
        )
    return wrapper_class


class PrioritizedSetting(object):
    """Return a value for a global setting according to configuration precedence.

    The following methods are searched in order for the setting:

    4. immediate values
    3. previously user-set value
    2. environment variable
    1. system setting
    0. implicit default

    If a value cannot be determined, a RuntimeError is raised.

    The ``env_var`` argument specifies the name of an environment to check for
    setting values, e.g. ``"AZURE_LOG_LEVEL"``.

    The optional ``system_hook`` can be used to specify a function that will
    attempt to look up a value for the setting from system-wide configurations.

    The optional ``default`` argument specified an implicit default value for
    the setting that is returned if no other methods provide a value.

    A ``convert`` agument may be provided to convert values before they are
    returned. For instance to concert log levels in environment variables
    to ``logging`` module values.

    """

    def __init__(self, name, env_var=None, system_hook=None, default=_Unset, convert=None):

        self._name = name
        self._env_var = env_var
        self._system_hook = system_hook
        self._default = default
        self._convert = convert if convert else lambda x: x
        self._user_value = _Unset

    def __repr__(self):
        # type () -> str
        return "PrioritizedSetting(%r)" % self._name

    def __call__(self, value=None):
        # type: (Any) -> Any
        """Return the setting value according to the standard precedence.

        :param time: value
        :type time: str or int or float or None (default: None)
        :returns: the value of the setting
        :rtype: str or int or float
        :raises: RuntimeError

        """

        # 4. immediate values
        if value is not None:
            return self._convert(value)

        # 3. previously user-set value
        if self._user_value is not _Unset:
            return self._convert(self._user_value)

        # 2. environment variable
        if self._env_var and self._env_var in os.environ:
            return self._convert(os.environ[self._env_var])

        # 1. system setting
        if self._system_hook:
            return self._convert(self._system_hook())

        # 0. implicit default
        if self._default is not _Unset:
            return self._convert(self._default)

        raise RuntimeError("No configured value found for setting %r" % self._name)

    def __get__(self, instance, owner):
        return self

    def __set__(self, instance, value):
        self.set_value(value)

    def set_value(self, value):
        # type: (Any) -> None
        """Specify a value for this setting programmatically.

        A value set this way takes precedence over all other methods except
        immediate values.

        :param time: a user-set value for this setting
        :type time: str or int or float
        :returns: None

        """
        self._user_value = value

    def unset_value(self):
        # () -> None
        """Unset the previous user value such that the priority is reset."""
        self._user_value = _Unset

    @property
    def env_var(self):
        return self._env_var

    @property
    def default(self):
        return self._default


class Settings(object):
    """Settings for globally used Azure configuration values.

    You probably don't want to create an instance of this class, but call the singleton instance:

    .. code-block:: python

        from azure.common.settings import settings
        settings.log_level = log_level = logging.DEBUG

    The following methods are searched in order for a setting:

    4. immediate values
    3. previously user-set value
    2. environment variable
    1. system setting
    0. implicit default

    An implicit default is (optionally) defined by the setting attribute itself.

    A system setting value can be obtained from registries or other OS configuration
    for settings that support that method.

    An environment variable value is obtained from ``os.environ``

    User-set values many be specified by assigning to the attribute:

    .. code-block:: python

        settings.log_level = log_level = logging.DEBUG

    Immediate values are (optionally) provided when the setting is retrieved:

    .. code-block:: python

        settings.log_level(logging.DEBUG())

    Immediate values are most often useful to provide from optional arguments
    to client functions. If the argument value is not None, it will be returned
    as-is. Otherwise, the setting searches other methods according to the
    precedence rules.

    Immutable configuration snapshots can be created with the following methods:

    * settings.defaults returns the base defaultsvalues , ignoring any environment or system
      or user settings

    * settings.current returns the current computation of settings including prioritizatiom
      of configuration sources, unless defaults_only is set to True (in which case the result
      is identical to settings.defaults)

    * settings.config can be called with specific values to override what settings.current
      would provide

    .. code-block:: python

        # return current settings with log level overridden
        settings.config(log_level=logging.DEBUG)

    :cvar log_level: a log level to use across all Azure client SDKs (AZURE_LOG_LEVEL)
    :type log_level: PrioritizedSetting
    :cvar tracing_enabled: Whether tracing should be enabled across Azure SDKs (AZURE_TRACING_ENABLED)
    :type tracing_enabled: PrioritizedSetting
    :cvar tracing_implementation: The tracing implementation to use (AZURE_SDK_TRACING_IMPLEMENTATION)
    :type tracing_implementation: PrioritizedSetting

    :Example:

    >>> import logging
    >>> from azure.core.settings import settings
    >>> settings.log_level = logging.DEBUG
    >>> settings.log_level()
    10

    >>> settings.log_level(logging.WARN)
    30

    """

    def __init__(self):
        self._defaults_only = False

    @property
    def defaults_only(self):
        """Whether to ignore environment and system settings and return only base default values.

        :rtype: bool
        """
        return self._defaults_only

    @defaults_only.setter
    def defaults_only(self, value):
        self._defaults_only = value

    @property
    def defaults(self):
        """Return implicit default values for all settings, ignoring environment and system.

        :rtype: namedtuple
        """
        props = {k: v.default for (k, v) in self.__class__.__dict__.items() if isinstance(v, PrioritizedSetting)}
        return self._config(props)

    @property
    def current(self):
        """Return the current values for all settings.

        :rtype: namedtuple
        """
        if self.defaults_only:
            return self.defaults
        return self.config()

    def config(self, **kwargs):
        """ Return the currently computed settings, with values overridden by parameter values.

        Examples:

        .. code-block:: python

           # return current settings with log level overridden
           settings.config(log_level=logging.DEBUG)

        """
        props = {k: v() for (k, v) in self.__class__.__dict__.items() if isinstance(v, PrioritizedSetting)}
        props.update(kwargs)
        return self._config(props)

    def _config(self, props):  # pylint: disable=no-self-use
        Config = namedtuple("Config", list(props.keys()))
        return Config(**props)

    log_level = PrioritizedSetting(
        "log_level", env_var="AZURE_LOG_LEVEL", convert=convert_logging, default=logging.INFO
    )

    tracing_enabled = PrioritizedSetting(
        "tracing_enbled", env_var="AZURE_TRACING_ENABLED", convert=convert_bool, default=False
    )

    tracing_implementation = PrioritizedSetting(
        "tracing_implementation", env_var="AZURE_SDK_TRACING_IMPLEMENTATION", convert=convert_tracing_impl, default=None
    )


settings = Settings()
"""The settings unique instance.

:type settings: Settings
"""
